The basic concept of utilizing a positive temperature coefficient resistance heater is disclosed in Sandford et al U.S. Pat. No. 3,410,984. The Sandford et al patent discloses an electric blanket in which the blanket wire for distributing heat over the blanket surface includes a pair of low resistance conductors which are spaced apart by the positive temperature coefficient material which serves as the heater for the blanket. The PTC material in the Sandford et al patent is described as a polyethylene which has dispersed in it electrically conductive particles such as carbon black to provide the desired characteristics in which the resistance of the material increases with increasing temperature.
Electric blankets have typically been constructed including some sort of flexible fabric shell which is formed with pockets or passageways disposed in parallel, spaced relation and extending lengthwise of the blanket shell. These passageways receive an elongated heating wire which serves to deliver heat to the entire area of the blanket when the blanket is connected to a suitable source of power. One of the basic problems which has faced blanket designers is that of providing means of sensing overheat conditions in order that the risk of fire and injury may be minimized. The basis of the problem is the fact that a blanket which will heat evenly when laid flat and either uncovered or covered uniformly will tend to have dangerous overheat conditions if the blanket is bunched or folded so that heat delivered to local areas is not dissipated at the same rate as the same heat in other areas. The continuing of these overheat conditions often results in attaining a temperature at which the insulation on the electric heater breaks down or combustion of the blanket material occurs.
In order to overcome or prevent such local overheat conditions, electric blankets have been provided with various types of temperature sensing means to deactivate or disable portions of the blanket involved in the overheat conditions. One approach has been to provide a number of series connected discrete bimetallic thermostats throughout the length of the blanket heater wire. These thermostats are closely spaced enough to sense most overheating that may develop. Another type of prior art heater involves a sensing wire fabricated with the heating wire or positioned closely adjacent thereto to provide a continuous sensing of the temperature conditions along the heating element wire. Such a sensing element or wire utilizes a positive or negative temperature coefficient of resistance material between two spaced conductors. By monitoring the overall resistance of the wire, it is possible to sense an overheat condition and deactivate the blanket. The approach followed in the Sandford et al patent referred to above represents an improvement over the sensor wire approach in that the heating element wire itself is self-limiting so that any local overheat condition causes an increase in the resistance of the heating element material in that area thereby reducing the portion of the power delivered to that part of the blanket and lessening the tendency of the overheat condition to continue.
Although the Sandford et al patent issued more than ten years ago, there have been no electric blankets marketed utilizing the type of PTC heating element wire disclosed therein. The reasons relate to the problems in fabricating a wire which will have the proper resistance characteristics as the temperature of the wire is varied. There are also problems in producing positive temperature coefficient resistance materials which will have uniform resistance characteristics over the expected life of the product. Certain aging conditions have a tendency to cause wide fluctuation in the resistance characteristics. In an effort to resolve some of these resistance and aging problems, various alternative approaches have been taken to the PTC material. The composition of the material as well as the manner of processing, including annealing, has evolved considerably. The patents to Bedard et al U.S. Pat. No. 3,858,144 and Bedard et al U.S. Pat. No. 3,914,363 are noted in connection with the disclosure of various approaches to the fabrication of the PTC wire. The above noted Bedard et al patents recognize a need for annealing the PTC wire at a temperature at or above the melting point of the PTC material. This condition necessitates the use of some sort of thermoplastic coating material around the PTC material to maintain the integrity of the PTC wire during the annealing process.
Another problem which is encountered is that of preventing short-circuiting of the conductors during the annealing process since the PTC material which holds the conductors in spaced relation tends to soften considerably and the conductors themselves are often mechanical stresses which tend to cause the conductors to shift their relative spacing within the PTC material. In this connection, the patent to Smith-Johannsen U.S. Pat. No. 3,793,716 is noted. The Smith-Johannsen patent shows the use of a braided envelope which is placed around the conductors to prevent contact between the conductors and to control their separation. Another patent of interest with respect to the use of separators is Crowley U.S. Pat. No. 2,820,085.
Another problem involved in the proper design of a PTC blanket wire is that of achieving satisfactory flexibility in the wire. Most of the prior art work with respect to self-limiting PTC heater wires has been done in connection with wire for use in industrial equipment wire. The wire is wrapped around pipes or similar pieces of equipment. However, in an electric blanket, it is necessary that the wire be very small in cross-section and flexible so as to permit the blanket shell in which the heating element is located to be flexed and folded easily.